IN A Newtonian telescope either a right-angle prism or a diagonal mirror of plane-parallel glass serves to turn the light focused by the concave mirror at right angles into the eyepiece. If a mirror is used, its front surface must be aluminized or only about four per cent of the light will reach the eye, the remainder passing through the diagonal glass to be wasted. When observing the sun, however, the large mirror of the telescope collects far more light than the eye can accept with comfort. Then an unaluminized diagonal that wastes light in this manner actually becomes desirable. The difficulty in using such a diagonal is that a fraction of the light is reflected back from the inside of the rear surface and reappears at the front to form a second image of the sun nearly as bright as the primary one beside it.

"Obviously," writes John M. Holeman of Richland, Wash., "what is needed is a piece of glass having only one side. A more attainable alternative is to use a piece of glass having its second side tilted with respect to the first, or front, side at an angle large enough to throw the secondary image out of the field of view of the lowest-powered eyepiece to be used. Since the field of view of most telescopes is narrow, an angle of 10 degrees between the faces of the prism is adequate for most conditions. This prism is known as the Herschel wedge or solar diagonal. The following notes describe how one was made.

"The optical requirements of this prism are of the simplest. The first surface should be flat to a quarter-wavelength and well polished. The figure of the second surface is not important, since the image formed here is not to be used. The other surfaces may be left ground. This is one of the easiest prisms to make, and a good first project for the beginner in prism making.

"The prism can be made from a rectangle of plate glass if a piece with one side flat enough can be found. If optical glass is desired a tank prism can be bought and cut down. Salvage companies furnish 90-degree prisms that have entrance and exit faces 1 1/2 inches wide and 6 inches long. The figure usually found on tank prisms is turned-down edge and ends. The faces are usually convex, one nearly flat, and the third may be anything. The flatness of the faces can be tested with interference fringes, and the best one chosen for the first surface of the solar diagonal.

"After the best surface has been marked off with a wax pencil, it is covered with painter's tape or beeswax and the prism is cut crosswise to leave a rectangular face of the desired dimensions, as shown in the upper left-hand part of the drawing below. [The drawing was made to illustrate this article by Russell W. Porter shortly before his death.] This may be done by hand with a hacksaw blade and coarse Carborundum grains.

"Next the prism is cut through at an angle to make a second face at about 10 degrees to the first. The rough sides are trued up and ground smooth, the first surface remaining protected by tape or wax. A stove lid or a piece of glass may be used as the tool, and the prism handground on it with Carbo and water. A high-grade prism may, if preferred, be made by the special method described by Fred Ferson in Amateur Telescope Making–Advanced (printings since mid 1944) and then polishing as he suggests in a glass surround. This will produce a beautiful prism with truly flat sides which is really unnecessary for the purpose. The surfaces and ends may also be ground and polished by the same means if one is quixotic enough to put in an additional 42 hours, as I did.

"There is so much opportunity for originality in making the metal parts of the accessory, and so much depends upon the equipment available to the individual, that a detailed description of a method would be useless except to a few.

"The mounting needs a tubular extension in the end that fits into the telescope where the eyepiece normally goes (the left-hand end in the illustration) and a holder at right angles to it to take the eyepiece. Between these, and at the vertex of the right angle, should be the first face of the solar wedge. This should be tilted at 45 degrees, with the thick base toward the eyepiece holder. The rear of the housing should be left open to allow the direct light and heat of the sun to escape. The simplest means of supporting the wedge is to put it behind a cut-out plate of metal slanted at the proper angle and held in place by a retainer of wood, as shown in the drawing. The completed assembly is shown at the upper right.

"Besides the prism, a dark filter is needed at the eyepiece. For the sun a war-surplus dark neutral filter (No. 605) may be had from a salvage company. For the moon, where there is also an excess of light, the light neutral filter No. 604 is excellent. It is desirable to mount the dark filter as close as possible to the eye lens of the eyepiece, so that the eye can reach the necessary eyepoint, which is pretty close in most telescope eyepieces. This is usually done by burnishing the filter into a thin circular cell like the left-hand one of the two in the lower right-hand quarter of the drawing. This cell is substituted for the screw cap of the eyepiece. A setscrew in the edge will keep it from falling off. The technique of burnishing or crimping in the filter is the same as for a lens, and is described by Porter in Amateur Telescope Making, page 70: a thin edge is turned up and rolled over against the glass with a small roller while the lathe is running at its fastest speed. In Lens Work for Amateurs Henry Orford very briefly describes this process and shows two lathe tools for burnishing, one a cutting tool with profile that excavates a hollow near the edge, the other a little roller. However, the job may be done with a round-ended tool having no roller. "If the variable filter shown in the lower right-hand corner of the drawing is preferred, it can be made from two small Polaroid filters No. 688. If one of these remains stationary and the other is rotated on top of it, the light will diminish gradually from about 35 per cent to practically zero. One way to mount this is to burnish one Polaroid into a rimmed cell, as with the simple filter, and to burnish the second Polaroid into a thin disk fitting into the cavity in the first cell. A retainer sprung or screwed in place keeps the whole assembly together. Rotation of the inside filter is obtained by 3 screw moving in a slot in the rim of the outer cell and threaded into the inner disk. The protruding head of the screw serves to rotate the filters."

After Holeman wrote the preceding article and Porter illustrated it, a question arose whose largely negative answer may nevertheless have positive value. This concerns possible injury to the eye of the observer from continued use of the second of the two types of filters described: the variable density filter.

During the war the Polaroid Corporation developed its crossed Polaroid variable filter attachment that was and still is used in large numbers of military binoculars. These have virtually complete absorption for infrared and ultraviolet radiation, and may be used with safety on telescopes of any aperture–for the observation for which they were designed. They were not, however, designed for direct observation of the solar disk, and they should not be required to protect the observer under such conditions.

Preceding the filter in the optical train, with the telescope aimed squarely at the sun, is an objective lens or mirror of 6-inch or larger aperture. This gathers as much radiation as would a burning glass of the same diameter. Next is a Herschel wedge that throws 95 per cent of this radiation out of the train. But the 5 per cent that remains and passes to the filter is still far more than a filter is called upon to absorb in other kinds of observation. It is doubtful whether the variable density filter has enough absorption in the infrared to safeguard the observer while he examines the solar image, as he might, for several minutes at a time, day after day, year after year. Yet the fact that the visible light is cut down can make him think he is safe, while the invisible infrared radiation may be cooking his eye.

This in no way bears upon the use of binoculars equipped with variable density filters even in prolonged terrestrial use, nor does it bear upon Polaroid sunglasses. These absorb the ultraviolet, while in normal daylight the infrared is insignificant–except when gazing directly at the sun, which our senses automatically forbid anyway. The consideration applies only to variable density filters (1) when used directly on the sun's disk (2) for prolonged observation (3) with a large light-collector.

In Sky and Telescope, May, 1950, Leo J. Scanlon described valuable experience with the solar-filter problem. After many experiments he found most satisfactory a telescope equipped with a Herschel wedge and, over the objective, an opaque diaphragm cut out to permit insertion of welder's filter lenses 2 inches by 4 and in several available densities.

If instead of the preceding arrangements the objective is itself reduced to small aperture, the resolving power is correspondingly reduced, while the alternative of a narrow circular opening at the edge conduces to diffraction effects.

DAVE BROADHEAD of Wellsville, N. Y., has devised a greatly improved "cookie cutter" for the purpose of cutting disks from larger pieces of glass with abrasive grains. It is shown in the illustration on this page, another drawing made by Porter shortly before his death. Its principal feature is an up-and-down pumping motion that continually flushes the broken-down abrasive and glass away from the edge of the cutter, where it would otherwise clog the groove and greatly impede the rolling of abrasive grains that do the cutting. This improvement was inspired by recollection of the "Easy" washing machine. By its use cutting was so greatly increased in speed that Broadhead was able to cut 10 1/2-inch disks out of 12 1/2-inch Pyrex telescope blanks 2 1/4 inches thick in only 2 1/2 hours.

The drawing shows a 12 1/2-inch Pyrex blank, on top of which is a ring of iron pipe sealed to the blank with wax. The ring might equally well be made of any kind of material. Inside it, with a narrow space between the two, is the cookie cutter of thin metal rotated by a central shaft.

"I made this cookie-cutter airtight," Broadhead writes, "carefully sealing all its cracks with wax. Then I poured enough coarse abrasive grains on the center of the glass to spread out to a depth of perhaps 3/8-inch, lowered the cutter on the glass and filled the annular space with water. As the shaft was raised and lowered a fraction of an inch, at the rate of about once a second, water was forced in and out around the cutting edge. This kept the pasty gunk of broken-down glass and abrasive constantly flushed out of the work, and carried new abrasive to it. The gunk particles suspended in the water flow with it to the overflow pipe as water is poured at intervals into the funnel.

"On disks of 10 1/2-inch diameter no small amount of force was needed for lifting the cookie-cutter and the column of air above it. This was applied by a 4-foot lever consisting of a 2-by-6 plank having a deep notch, or prongs, at one end. The plank was inserted over a fulcrum and under the drive pulley at the top of the vertical shaft and pumped up and down through a short amplitude. Obviously the force necessary to lift the cookie-cutter against the weight of the atmosphere would lift the Pyrex blank and all from the bench, if it were not firmly attached by clamps or otherwise.

"A part of the secret of success," Broadhead writes, "is the narrowness of the space between the cutter and the ring. Otherwise much of the unused abrasive that finds its way to this outer space will lodge there and not be used." Enough heat is generated after a time to melt the wax if the work is pushed too eagerly.

THERE IS NO essential difference between cookie-cutting the circumference of a disk and perforating it centrally for a Cassegrainian. In fact, the perforation in one big disk is sometimes as large as the outside diameter of anther. To supplement the preceding notes on cookie-cutters, A. H. Johns of Larchmont, N. Y., who was known to be making two 16-inch Cassegrainian mirrors for others as a part of his avocation, was invited to describe his methods of trepanning, using a length of tubing as the trepan. He writes:

"I had previously drilled 1-inch and 2-inch perforations in mirrors, and had no trouble when I, came to perforate the two16-inch disks 3 inches thick with holes 4 inches in diameter. These disks weigh about 50 pounds, which permitted dispensing with holding devices other than several thicknesses of wet newspaper between the disks and the bench on which they rested. These gave an excellent grip.

"The steel drilling tube had walls 1/8-inch thick. It must be checked to assure that the walls are true, since a bulge or a taper will spell disaster.

"I did not pitch or wax a cover glass to the face of the disk, as is sometimes done to avoid splintering the glass at the entrance point. If the drill is held sufficiently rigid and the whole system is properly squared up, there will be little likelihood of such splintering. This rigidity also safeguards against the drill running down at an angle.

"Because my disks were only rough-ground at the perforating stage, and also because the hole to be cut was fairly large, I used No. 80 Carbo. This left a few pits around the edge of the perforation, but these were later eliminated in grinding the mirror curve.

"To prevent gunk from splashing, a fence of heavy paper was wrapped around the edge of the disk. The drill was run at about 200 revolutions per minute, but there is nothing compulsory about this. For a 2-inch hole I once ran at 540 revolutions per minute, according to my log records. Plenty of Carbo and water were worked into the crevice and the grinding noise was maintained at maximum. The drill was lifted frequently, while running, to bring up the gunk and permit clean fresh Carbo to reach the bottom of the cut. When it began to pile up, this gunk was sponged away. It is not necessary to flood the surface with water.

"I ran the drill to within 1/4-inch of the back, filled the cut with wax (to be left while the mirror is processed) and inverted the disk to complete it by drilling from the back. If at this point the planes of the inverted disk-faces are not made parallel with their original planes, there will be difficulty in making the cuts meet.

"Splintering occurs only where the drill emerges and not at the entrance face. Were I planning to drill straight through the disk in a single operation, and were the splintered back-face objectionable, I would first pitch or wax a cover glass over the exit area.

"If strains are already present in the glass, drilling after polishing will warp the figure."

Suppliers and Organizations

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